It is common practice in compressed air systems to pass the pressurized air through an air dryer prior to being delivered to a storage reservoir or the like. The air dryer, such as, a C-1 air dryer, manufactured and sold by the Westinghouse Air Brake Company of Spartanburg, S.C., not only extracts water vapor, but also removes other contaminants entrained in the compressed air. The collected contaminants must be periodically drained from the C-1 type of air dryer. In order to remove the condensates, the air dryers are frequently provided with a normally closed pneumatically operated drain valve. In practice, the drain valve is opened during the compressor "OFF" cycle to permit the flow of purge air to atmosphere, and exhausts the discharge system of the air compressor unit. Presently, a D-4 type of pneumatic activated drain valve is used in combination with the C-1 air dryer. The existing drain valve employs a piston-stem assembly in which a brass piston is attached to a stainless steel stem. The piston includes a pair of annular grooves for receiving rubber O-rings which slide within a stainless steel bushing which is press-fitted within the housing or casing of the drain valve. The remote end of the stem carries a valve seal which is cooperatively associated with a valve seat located within the housing. A compression biasing spring engages the piston member and normally urges the valve seal toward the valve seat to a closed position when the compressor is turned ON. When the compressor is turned OFF, a pressure chamber is pressurized to cause the linear movement of the piston stem assembly to open the valve seat and to purge the moisture vapor from the air dryer. It has been found that there is a leakage and seizure problem with these previous drain valves. The lack of lubrication causes wear on the rubber O-rings which results in leakage. In addition, the absence of lubricity results in high static and dynamic frictional forces which act against the spring loading and therefore the valve seal does not completely and tightly seal against the valve seat so that there is a constant air seepage which can result in the continuous operation of the air compressor.